Ubiquitous Cell-Free Massive MIMO Communications
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The concept of delivering widespread wireless communication services by splitting a coverage area into many cells dates back to the 1940s and the first practical cellular networks were trialled in the 1970s. Since then, we have witnessed an incredible wireless revolution, where cellular connectivity has gone from being a luxury product to a commodity and where the main application has changed from voice calls to data transfer. From 1G to 4G, the massive traffic growth has been managed by a combination of wider bandwidths, refined radio interfaces, and smaller cells [1]. Due its cost-efficiency, the latter has contributed the most; cell towers used to be 10 km apart and are now only 100 m apart in urban areas. While rigid frequency planning was originally used to alleviate inter-cell interference, the interference is becoming a major bottleneck as we densify the networks [2], because each cell is then hit by non-negligible interference from many neighboring cells. One way to deal with inter-cell interference is to manage it spatially by Massive MIMO (multiple-input multiple-output) methods [3], which is a key 5G technology. However, looking further ahead, we need a more radical approach; we need to challenge the cellular paradigm and realize that inter-cell interference is not a fundamental phenomenon but the result of the cell boundaries that we have purposely created in the data transmission.
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